Flow-dependent vascular heat transfer during microwave thermal ablation

Current perspectives on microwave ablation of liver lesions in difficult locations

Microwave ablation (MWA) is becoming the standard of care in treating liver lesions smaller than 3 cm benefiting from a plethora of radiofrequency ablation (RFA) data in the literature. Some of the advantages of MWA compared to RFA are as follows: Faster ablations, more reproducible and predictable heating, better thermal conductivity in different liver tissue environments, and less susceptibility to heat-sink effect. Despite its many advantages, there are still concerns regarding MWA use in high-risk locations such as near portal veins, near the bile ducts, and near the heart. Some centers have historically considered these tumor locations as a contraindication to percutaneous thermal ablation. In this review, we summarize the current data on the safety of MWA of liver tumors in challenging locations. We also discuss several technical tips with examples provided.

Minimally Invasive Interventional Procedures for Metastatic Bone Disease: A Comprehensive Review

Metastases are the main type of malignancy involving bone, which is the third most frequent site of metastatic carcinoma, after lung and liver. Skeletal-related events such as intractable pain, spinal cord compression, and pathologic fractures pose a serious burden on patients’ quality of life. For this reason, mini-invasive treatments for the management of bone metastases were developed with the goal of pain relief and functional status improvement. These techniques include embolization, thermal ablation, electrochemotherapy, cementoplasty, and MRI-guided high-intensity focused ultrasound. In order to achieve durable pain palliation and disease control, mini-invasive procedures are combined with chemotherapy, radiation therapy, surgery, or analgesics. The purpose of this review is to summarize the recently published literature regarding interventional radiology procedures in the treatment of cancer patients with bone metastases, focusing on the efficacy, complications, local disease control and recurrence rate.

Dielectric Properties of Ovine Heart at Microwave Frequencies

Accurate knowledge of the dielectric properties of biological tissues is important in dosimetry studies and for medical diagnostic, monitoring and therapeutic technologies. In particular, the dielectric properties of the heart are used in numerical simulations of radiofrequency and microwave heart ablation. In one recent study, it was demonstrated that the dielectric properties of different components of the heart can vary considerably, contrary to previous literature that treated the heart as a homogeneous organ with measurements that ignored the anatomical location. Therefore, in this study, we record and report the dielectric properties of the heart as a heterogeneous organ. We measured the dielectric properties at different locations inside and outside of the heart over the 500 MHz to 20 GHz frequency range. Different parts of the heart were identified based on the anatomy of the heart and their function; they include the epicardium, endocardium, myocardium, exterior and interior surfaces of atrial appendage, and the luminal surface of the great vessels. The measured dielectric properties for each part of the heart are reported at both a single frequency (2.4 GHz), which is of interest in microwave medical applications, and as parameters of a broadband Debye model. The results show that in terms of dielectric properties, different parts of the heart should not be considered the same, with more than 25% difference in dielectric properties between some parts. The specific Debye models and single frequency dielectric properties from this study can be used to develop more detailed models of the heart to be used in electromagnetic modeling.

Effect of microwave ablation treatment of hepatic malignancies on serum cytokine levels

Background

Microwave ablation (MWA) is widely used to treat unresectable primary and secondary malignancies of the liver, and a limited number of studies indicate that ablation can cause not only necrosis at the in situ site but also an immunoreaction of the whole body. This study aimed to investigate the effects of MWA on cytokines in patients who underwent MWA for a hepatic malignancy.

Methods

Patients admitted to the Oncology Department in the First Affiliated Hospital of Soochow University between June 2015 and February 2019 were selected. Peripheral blood was collected from patients with a hepatic malignancy treated with MWA. The levels of cytokines (IL-2, IFN-γ, TNF-α, IL-12 p40, IL-12 p70, IL-4, IL-6, IL-8, IL-10, and vascular endothelial growth factor (VEGF)) were detected with a Milliplex® MAP Kit. The comparison times were as follows: before ablation, 24 h after ablation, 15 days after ablation, and 30 days after ablation. Data were analyzed using a paired sample t-tests and Spearman’s correlation analysis.

Results

A total of 43 patients with hepatic malignancies were assessed. There were significant differences in IL-2, IL-12 p40, IL-12 p70, IL-1β, IL-8, and TNF-α at 24 h after MWA. Significant increases (> 2-fold vs. before ablation) were observed in IL-2, IL-1β, IL-6, IL-8, IL-10, and TNF-α after MWA. Elevated IL-2 and IL-6 levels after ablation were positively correlated with energy output during the MWA procedure.

Conclusions

WA treatment for hepatic malignancies can alter the serum levels of several cytokines such as IL-2 and IL-6.

4D Flow MR Imaging to Improve Microwave Ablation Prediction Models: A Feasibility Study in an In Vivo Porcine Liver

PURPOSE

To characterize the effect of hepatic vessel flow using 4-dimensional (4D) flow magnetic resonance (MR) imaging and correlate their effect on microwave ablation volumes in an in vivo non-cirrhotic porcine liver model.

MATERIALS AND METHODS

Microwave ablation antennas were placed under ultrasound guidance in each liver lobe of swine (n = 3 in each animal) for a total of 9 ablations. Pre- and post-ablation 4D flow MR imaging was acquired to quantify flow changes in the hepatic vasculature. Flow measurements, along with encompassed vessel size and vessel-antenna spacing, were then correlated with final ablation volume from segmented MR images.

RESULTS

The linear regression model demonstrated that the preablation measurement of encompassed hepatic vein size (β = -0.80 ± 0.25, 95% confidence interval [CI] -1.15 to -0.22; P = .02) was significantly correlated to final ablation zone volume. The addition of hepatic vein flow rate found via 4D flow MRI (β = -0.83 ± 0.65, 95% CI -2.50 to 0.84; P = .26), and distance from antenna to hepatic vein (β = 0.26 ± 0.26, 95% CI -0.40 to 0.92; P = .36) improved the model accuracy but not significantly so (multivariate adjusted R² = 0.70 vs univariate (vessel size) adjusted R² = 0.63, P = .24).

CONCLUSIONS

Hepatic vein size in an encompassed ablation zone was found to be significantly correlated with final ablation zone volume. Although the univariate 4D flow MR imaging-acquired measurements alone were not found to be statistically significant, its addition to hepatic vein size improved the accuracy of the ablation volume regression model. Pre-ablation 4D flow MR imaging of the liver may assist in prospectively optimizing thermal ablation treatment.

Can we ablate liver lesions close to large portal and hepatic veins with MR-guided HIFU? An experimental study in a porcine model

OBJECTIVES

Invasive treatment of tumors adjacent to large hepatic vessels is a continuous clinical challenge. The primary aim of this study was to examine the feasibility of ablating liver tissue adjacent to large hepatic and portal veins with magnetic resonance imaging-guided high-intensity focused ultrasound (MRgHIFU). The secondary aim was to compare sonication data for ablations performed adjacent to hepatic veins (HV) versus portal veins (PV).

MATERIALS AND METHODS

MRgHIFU ablations were performed in six male land swine under general anesthesia. Ablation cells of either 4 or 8 mm diameter were planned in clusters (two/animal) adjacent either to HV (n = 6) or to PV (n = 6), with diameter ≥ 5 mm. Ablations were made using 200 W and 1.2 MHz. Post-procedure evaluation was made on contrast-enhanced MRI (T1w CE-MRI), histopathology, and ablation data from the HIFU system.

RESULTS

A total of 153 ablations in 81 cells and 12 clusters were performed. There were visible lesions with non-perfused volumes in all animals on T1w CE-MRI images. Histopathology showed hemorrhage and necrosis in all 12 clusters, with a median shortest distance to vessel wall of 0.4 mm (range 0-2.7 mm). Edema and endothelial swelling were observed without vessel wall rupture. In 8-mm ablations (n = 125), heat sink was detected more often for HV (43%) than for PV (19%; p = 0.04).

CONCLUSIONS

Ablations yielding coagulative necrosis of liver tissue can be performed adjacent to large hepatic vessels while keeping the vessel walls intact. This indicates that perivascular tumor ablation in the liver is feasible using MRgHIFU.

KEY POINTS

• High-intensity focused ultrasound ablation is a non-invasive treatment modality that can be used for treatment of liver tumors. • This study shows that ablations of liver tissue can be performed adjacent to large hepatic vessels in an experimental setting. • Liver tumors close to large vessels can potentially be treated using this modality.

Percutaneous Irreversible Electroporation (IRE) of Hepatic Malignancy: A Bi-institutional Analysis of Safety and Outcomes

Aim

Irreversible electroporation (IRE) is a non-thermal ablative option in patients unsuitable for standard thermal ablation, due to its potential to preserve collagenous structures (vessels and ducts) and a reduced susceptibility to heat sink effects. In this series from two large tertiary referral hepatobiliary centres, we aim to assess the safety/outcomes of hepatic IRE.

Materials and Methods

Bi-institutional retrospective, longitudinal follow-up series of IRE for primary hepatic malignancy; [hepatocellular carcinoma (n = 20), cholangiocarcinoma (n = 3)] and secondary metastatic disease; colorectal (n = 28), neuroendocrine (n = 1), pancreatic (n = 1), breast (n = 1), gastrointestinal stromal tumour (GIST, n = 1) and malignant thymoma (n = 1). Outcome measures included procedural safety/effectiveness, time to progression and time to death.

Results

Between 2013 and 2017, 52 patients underwent percutaneous IRE of 59 liver tumours in 53 sessions. All tumours were deemed unsuitable for thermal ablation. Cases were performed using ultrasound (US) or computed tomography (CT) guidance. A complete ablation was achieved in n = 44, (75%) of cases with an overall complication rate of 17% (n = 9). Of the complete ablation group, median time to progression was 8 months. At 12 months, 44% were progression-free (95% CI 30–66%). The data suggest that larger lesion size (> 2 cm) is associated with shorter time to progression and there is highly significant difference with faster time to progression in mCRC compared with HCC. Median survival time was 38 months.

Conclusion

This bi-institutional review is the largest UK series of IRE and suggests this ablative technology can be a useful tool, but appears to mainly induce local tumour control rather than cure with HCC having better outcomes than mCRC.

Hydrodissection of the Retrohepatic Space: A Technique to Physically Separate a Liver Tumour from the Inferior Vena Cava and the Ostia of the Hepatic Veins

OBJECTIVE

To report a technique of percutaneous retrohepatic hydrodissection, highlighting its potential to physically separate liver tumours from the inferior vena cava (IVC) and the ostia of the hepatic veins (HV).

MATERIALS AND METHODS

Between December 2017 and April 2018, hydrodissection of the retrohepatic IVC was performed in 5 patients (5 females; mean age 64.5 years) undergoing percutaneous ablation of 5 liver metastases (mean size: 3.6 cm) located adjacent to the IVC. Number of hydrodissection needles, volume of hydrodissection, separation of tumour/liver parenchyma from IVC/HV post-hydrodissection; technical success of ablation; and complications were tabulated.

RESULTS

Two to three 22G spinal needles were required per case for adequate dissection. Mean volume to obtain sufficient hydrodissection was 410 ml on average. Physical separation of the IVC and tumour/hepatic parenchyma was successful in all cases, by 9 mm on average (range 5-12 mm). It also leaded to physical separation of the ostia of the right and middle HV in all cases. There was no early or delayed complication, notably no venous thrombosis in the post-operative period. All lesions but one were completely ablated after one session at 3-month follow-up. The patient with residual tumour was successfully retreated.

CONCLUSION

Retrohepatic hydrodissection is a feasible technique to separate a tumour from the IVC and/or ostia of the HV. This could potentially limit the heat-sink effect/reduce the risk of thrombosis. Larger follow-up studies are required to assess efficacy on a long-term basis.

Potential Mechanisms of Vascular Thrombosis after Microwave Ablation in an in Vivo Liver

PURPOSE

To evaluate potential biologic and thermal mechanisms of the observed differences in thrombosis rates between hepatic vessels during microwave (MW) ablation procedures.

MATERIALS AND METHODS

MW ablation antennae were placed in single liver lobes of 2 in vivo porcine liver models (n = 3 in each animal; N = 6 total) in the proximity of a large (> 5 mm) portal vein (PV) and hepatic veins (HVs). Each ablation was performed with 100 W for 5 minutes. Conventional ultrasound imaging and intravascular temperature probes were used to evaluate vessel patency and temperature changes during the ablation procedure. Vascular endothelium was harvested 1 hour after ablation and used to characterize genes and proteins associated with thrombosis in PVs and HVs.

RESULTS

Targeted PVs within the MW ablation zone exhibited thrombosis at a significantly higher rate than HVs (54.5% vs 0.0%; P = .0046). There was a negligible change in intravascular temperature in PVs and HVs during the ablation procedure (0.2°C ± 0.4 vs 0.6°C ± 0.9; P = .46). PVs exhibited significantly higher gene expression than HVs in terms of fold differences in thrombomodulin (2.9 ± 2.0; P = .0001), von Willebrand factor (vWF; 7.6 ± 1.5; P = .0001), endothelial protein C receptor (3.50 ± 0.49; P = .0011), and plasminogen activator inhibitor (1.46 ± 0.05; P = .0014). Western blot analysis showed significantly higher expression of vWF (2.32 ± 0.92; P = .031) in PVs compared with HVs.

CONCLUSIONS

Large PVs exhibit thrombosis more frequently than HVs during MW ablation procedures. Biologic differences in thrombogenicity, rather than heat transfer, between PVs and HVs may contribute to their different rates of thrombosis.

Microwave ablation in primary and secondary liver tumours: technical and clinical approaches

Thermal ablation is increasingly being utilised in the treatment of primary and metastatic liver tumours, both as curative therapy and as a bridge to transplantation. Recent advances in high-powered microwave ablation systems have allowed physicians to realise the theoretical heating advantages of microwave energy compared to other ablation modalities. As a result there is a growing body of literature detailing the effects of microwave energy on tissue heating, as well as its effect on clinical outcomes. This article will discuss the relevant physics, review current clinical outcomes and then describe the current techniques used to optimise patient care when using microwave ablation systems.

Effects of Microwave Ablation on Arterial and Venous Vasculature after Treatment of Hepatocellular Carcinoma

Purpose To characterize vessel occlusion rates and their role in local tumor progression in patients with hepatocellular carcinoma (HCC) who underwent microwave tumor ablation. Materials and Methods This institutional review board approved, HIPAA-compliant retrospective review included 95 patients (75 men and 20 women) with 124 primary HCCs who were treated at a single center between January 2011 and March 2014. Complete occlusion of the portal veins, hepatic veins, and hepatic arteries within and directly abutting the ablation zone was identified with postprocedure contrast material-enhanced computed tomography. For each vessel identified in the ablation zone, its size and antenna spacing were recorded and correlated with vascular occlusion with logistic regression analysis. Local tumor progression rates were then compared between patent and occluded vessels for each vessel type with Fisher exact test. Results Occlusion was identified in 39.7% of portal veins (29 of 73), 15.0% of hepatic veins (six of 40), and 14.2% of hepatic arteries (10 of 70) encompassed within the ablation zone. Hepatic vein occlusion was significantly correlated with a smaller vessel size (P = .036) and vessel-antenna spacing (P = .006). Portal vein occlusion was only significantly correlated with a smaller vessel size (P = .001), particularly in vessels that were less than 3 mm in diameter. Local tumor progression rates were significantly correlated with patent hepatic arteries within the ablation zone (P = .02) but not with patent hepatic (P = .57) or portal (P = .14) veins. Conclusion During microwave ablation of HCC, hepatic veins and arteries were resistant to vessel occlusion compared with portal veins, and only arterial patency within an ablation zone was related to local tumor progression. ^© RSNA, 2016.

Liver Ablation: Best Practice

Tumor ablation in the liver has evolved to become a well-accepted tool in the management of increasing complex oncologic patients. At present, percutaneous ablation is considered first-line therapy for very early and early hepatocellular carcinoma and second-line therapy for colorectal carcinoma liver metastasis. Because thermal ablation is a treatment option for other primary and secondary liver tumors, an understanding of the underlying tumor biology is important when weighing the potential benefits of ablation. This article reviews ablation modalities, indications, patient selection, and imaging surveillance, and emphasizes technique-specific considerations for the performance of percutaneous ablation.

Percutaneous tumor ablation tools: microwave, radiofrequency, or cryoablation--what should you use and why?

Image-guided thermal ablation is an evolving and growing treatment option for patients with malignant disease of multiple organ systems. Treatment indications have been expanding to include benign tumors as well. Specifically, the most prevalent indications to date have been in the liver (primary and metastatic disease, as well as benign tumors such as hemangiomas and adenomas), kidney (primarily renal cell carcinoma, but also benign tumors such as angiomyolipomas and oncocytomas), lung (primary and metastatic disease), and soft tissue and/or bone (primarily metastatic disease and osteoid osteomas). Each organ system has different underlying tissue characteristics, which can have profound effects on the resulting thermal changes and ablation zone. Understanding these issues is important for optimizing clinical results. In addition, thermal ablation technology has evolved rapidly during the past several decades, with substantial technical and procedural improvements that can help improve clinical outcomes and safety profiles. Staying up to date on these developments is challenging but critical because the physical properties underlying the different ablation modalities and the appropriate use of adjuncts will have a tremendous effect on treatment results. Ultimately, combining an understanding of the physical properties of the ablation modalities with an understanding of the thermal kinetics in tissue and using the most appropriate ablation modality for each patient are key to optimizing clinical outcomes. Suggested algorithms are described that will help physicians choose among the various ablation modalities for individual patients.

Percutaneous tumor ablation tools: microwave, radiofrequency, or cryoablation--what should you use and why?

Image-guided thermal ablation is an evolving and growing treatment option for patients with malignant disease of multiple organ systems. Treatment indications have been expanding to include benign tumors as well. Specifically, the most prevalent indications to date have been in the liver (primary and metastatic disease, as well as benign tumors such as hemangiomas and adenomas), kidney (primarily renal cell carcinoma, but also benign tumors such as angiomyolipomas and oncocytomas), lung (primary and metastatic disease), and soft tissue and/or bone (primarily metastatic disease and osteoid osteomas). Each organ system has different underlying tissue characteristics, which can have profound effects on the resulting thermal changes and ablation zone. Understanding these issues is important for optimizing clinical results. In addition, thermal ablation technology has evolved rapidly during the past several decades, with substantial technical and procedural improvements that can help improve clinical outcomes and safety profiles. Staying up to date on these developments is challenging but critical because the physical properties underlying the different ablation modalities and the appropriate use of adjuncts will have a tremendous effect on treatment results. Ultimately, combining an understanding of the physical properties of the ablation modalities with an understanding of the thermal kinetics in tissue and using the most appropriate ablation modality for each patient are key to optimizing clinical outcomes. Suggested algorithms are described that will help physicians choose among the various ablation modalities for individual patients.

Microwave ablation energy delivery: influence of power pulsing on ablation results in an ex vivo and in vivo liver model

PURPOSE

The purpose of this study was to compare the impact of continuous and pulsed energy deliveries on microwave ablation growth and shape in unperfused and perfused liver models.

METHODS

A total of 15 kJ at 2.45 GHz was applied to ex vivo bovine liver using one of five delivery methods (n = 50 total, 10 per group): 25 W continuous for 10 min (25 W average), 50 W continuous for 5 min (50 W average), 100 W continuous for 2.5 min (100 W average), 100 W pulsed for 10 min (25 W average), and 100 W pulsed for 5 min (50 W average). A total of 30 kJ was applied to in vivo porcine livers (n = 35, 7 per group) using delivery methods similar to the ex vivo study, but with twice the total ablation time to offset heat loss to blood perfusion. Temperatures were monitored 5-20 mm from the ablation antenna, with values over 60 °C indicating acute cellular necrosis. Comparisons of ablation size and shape were made between experimental groups based on total energy delivery, average power applied, and peak power using ANOVA with post-hoc pairwise tests.

RESULTS

No significant differences were noted in ablation sizes or circularities between pulsed and continuous groups in ex vivo tissue. Temperature data demonstrated more rapid heating in pulsed ablations, suggesting that pulsing may overcome blood perfusion and coagulate tissues more rapidly in vivo. Differences in ablation size and shape were noted in vivo despite equivalent energy delivery among all groups. Overall, the largest ablation volume in vivo was produced with 100 W continuous for 5 min (265.7 ± 208.1 cm(3)). At 25 W average, pulsed-power ablation volumes were larger than continuous-power ablations (67.4 ± 34.5 cm(3) versus 23.6 ± 26.5 cm(3), P = 0.43). Similarly, pulsed ablations produced significantly greater length (P ≤ 0.01), with increase in diameter (P = 0.09) and a slight decrease in circularity (P = 0.97). When comparing 50 W average power groups, moderate differences in size were noted (P ≥ 0.06) and pulsed ablations were again slightly more circular.

CONCLUSIONS

Pulsed energy delivery created larger ablation zones at low average power compared to continuous energy delivery in the presence of blood perfusion. Shorter duty cycles appear to provide greater benefit when pulsing.

Heat sink effect on tumor ablation characteristics as observed in monopolar radiofrequency, bipolar radiofrequency, and microwave, using ex vivo calf liver model

Thermal ablation of liver tumors near large blood vessels is affected by the cooling effect of blood flow, leading to incomplete ablation. Hence, we conducted a comparative investigation of heat sink effect in monopolar (MP) and bipolar (BP) radiofrequency ablation (RFA), and microwave (MW) ablation devices.With a perfused calf liver, the ablative performances (volume, mass, density, dimensions), with and without heat sink, were measured. Heat sink was present when the ablative tip of the probes were 8.0 mm close to a major hepatic vein and absent when >30 mm away. Temperatures (T1 and T2) on either side of the hepatic vein near the tip of the probes, heating probe temperature (T3), outlet perfusate temperature (T4), and ablation time were monitored.With or without heat sink, BP radiofrequency ablated a larger volume and mass, compared with MP RFA or MW ablation, with latter device producing the highest density of tissue ablated. MW ablation produced an ellipsoidal shape while radiofrequency devices produced spheres.Percentage heat sink effect in Bipolar radiofrequency : Mono-polar radiofrequency : Microwave was (Volume) 33:41:22; (mass) 23:56:34; (density) 9.0:26:18; and (relative elipscity) 5.8:12.9:1.3, indicating that BP and MW devices were less affected.Percentage heat sink effect on time (minutes) to reach maximum temperature (W) = 13.28:9.2:29.8; time at maximum temperature (X) is 87:66:16.66; temperature difference (Y) between the thermal probes (T3) and the temperature (T1 + T2)/2 on either side of the hepatic vessel was 100:87:20; and temperature difference between the (T1 + T2)/2 and temperature of outlet circulating solution (T4), Z was 20.33:30.23:37.5.MW and BP radiofrequencies were less affected by heat sink while MP RFA was the most affected. With a single ablation, BP radiofrequency ablated a larger volume and mass regardless of heat sink.

Computational modelling of microwave tumour ablations

Microwave tissue heating is being increasingly utilised in several medical applications, including focal tumour ablation, cardiac ablation, haemostasis and resection assistance. Computational modelling of microwave ablations is a precise and repeatable technique that can assist with microwave system design, treatment planning and procedural analysis. Advances in coupling temperature and water content to electrical and thermal properties, along with tissue contraction, have led to increasingly accurate computational models. Developments in experimental validation have led to broader acceptability and applicability of these newer models. This review will discuss the basic theory, current trends and future direction of computational modelling of microwave ablations.